The present invention relates to optical filters and more particularly to an arrangement to separate incident radiation into a plurality of spectral bands.
In prior art arrangements the incident radiation is separated by a plurality of optical filters into a plurality of different spectral bands and each of the bands is detected by a different one of a corresponding plurality of detectors. The disadvantage of such an arrangement is the cost and reliability thereof.
It has been demonstrated that a layer of dielectric material, applied to the polished faces of an optical material, can change the reflection characteristics of the surface due to impedance mismatch with the surrounding medium. If the refractive index of the coating is intermediate the refractive index of the surrounding medium and the optical material or substrate, then the reflection is reduced (the principle of anti-reflection coatings). If the refractive index of the coating aggravates the mismatch, the reflection is enhanced. This matching effect varies with wavelength. It is greatest when the coating is an odd number of quarter wavelengths in optical thickness, and there is no effect when it is a multiple of one-half wavelength in optical thickness.
Employing multiple layers of dielectric material of alternate high and low refractive index can significantly enhance the reflectivity over a specified wavelength region.
Stacks of only a few quarter-wavelength layers using the high refractive index materials available in the infrared spectral region can have very high reflectance. For example, if the first medium is air with a refractive index n=1, the high index coating is Germanium (n=4), the low refractive index material is silicon monoxide (n=1.32), and the substrate is quartz, the following reflectance levels can be produced:
______________________________________ Number of Layers Reflectance ______________________________________ 1 0.701 2 0.962 3 0.998 ______________________________________
The use of sets of quarter-wavelength layers separated by half-wavelength spacer layers can produce the equivalent of multiple coupled circuits in an electronic filter.
Just as one can design a filter with a center frequency, specific passband and specific skirt selectivity, so can the multilayer interference filter be tailored.
Commercial filters have been produced from the near ultraviolet to the microwave region. In fact, a terminated transmission line or waveguide is directly analogous to the interference filter.
Interference filters are made by vacuum deposition of alternate layers of high and low refractive index material onto a suitable substrate. The substrate must, of course, transmit in the region in which the filter is desired to transmit.
In selecting material for the layers of the filter, the wavelength regions of good transmission, its hardness, freedom from contamination, ease of deposition, and temperature characteristics must be considered. Some candidate materials in the 3 to 5 micron region are Germanium, Zinc Sulfide, and Chiolite (sodium aluminum fluoride).